Patient follow-up is a major part of the daily clinical practice of the radiologist. To detect pathological changes as growing nodules, interstitial infiltrates and pleural effusions in the lungs, a current thorax image is compared with a previous recorded image. Temporal subtraction is a popular technique to aid the radiologist with this time consuming task. A previous image is subtracted from the current image after proper alignment and warping to visually enhance the pathological changes. Several studies have shown that a system with temporal subtraction significantly increases the detection rate of interval changes in digital chest radiographs. Studies have also shown that the technique also positively influences the radiologist's interpretation time. A few studies have applied temporal subtraction of CT images for cancer detection.
Nowadays, temporal subtraction of chest radiographs has made its entrance in commercially available CAD systems.
Prior to subtracting one image from the other, alignment of the corresponding anatomical structures is needed to remove irrelevant information from the subtraction image. Computed radiography or computed tomography images of the thorax suffer from non-rigid geometric distortions caused by the three dimensional (3D) displacement of corresponding structures due to differences in patient pose and inhalation, which pleads for the choice of a nonrigid registration algorithm. On the other hand, non-rigid warping has the unwanted effect of changing the size of a lesion. A tumor in a previous image might be blown up to match the tumor in the current image, such that no changes can be observed in the subtraction image. Hence, most authors use a warping technique which does not allow for large local deformations. For example, D. Loeckx et al., “Temporal subtraction of thorax CR images using a statistical deformation model”, IEEE Trans. Med. Imag. 22(11), pp. 1490-1504, 2003 applied a PCA deformation field which was trained on example deformations to capture inhalation and pose difference modes.